a history of science-2-第41部分

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


ed　the　ends　of　the　tube　to　exclude　the　dust；　seemed　to　attract　bits　of　paper　and　leaf…brass　as　well　as　the　glass　itself。　He　surmised　at　once　that　this　mysterious　electricity；　or　〃virtue；〃　as　it　was　called；　might　be　transmitted　through　other　substances　as　it　seemed　to　be　through　glass。　〃Having　by　me　an　ivory　ball　of　about　one　and　three…tenths　of　an　inch　in　diameter；〃　he　writes；　〃with　a　hole　through　it；　this　I　fixed　upon　a　fir…stick　about　four　inches　long；　thrusting　the　other　end　into　the　cork；　and　upon　rubbing　the　tube　found　that　the　ball　attracted　and　repelled　the　feather　with　more　vigor　than　the　cork　had　done；　repeating　its　attractions　and　repulsions　for　many　times　together。　I　then　fixed　the　ball　on　longer　sticks；　first　upon　one　of　eight　inches；　and　afterwards　upon　one　of　twenty…four　inches　long；　and　found　the　effect　the　same。　Then　I　made　use　of　iron；　and　then　brass　wire；　to　fix　the　ball　on；　inserting　the　other　end　of　the　wire　in　the　cork；　as　before；　and　found　that　the　attraction　was　the　same　as　when　the　fir…sticks　were　made　use　of；　and　that　when　the　feather　was　held　over　against　any　part　of　the　wire　it　was　attracted　by　it；　but　though　it　was　then　nearer　the　tube；　yet　its　attraction　was　not　so　strong　as　that　of　the　ball。　When　the　wire　of　two　or　three　feet　long　was　used；　its　vibrations；　caused　by　the　rubbing　of　the　tube；　made　it　somewhat　troublesome　to　be　managed。　This　put　me　to　thinking　whether；　if　the　ball　was　hung　by　a　pack…thread　and　suspended　by　a　loop　on　the　tube；　the　electricity　would　not　be　carried　down　the　line　to　the　ball；　I　found　it　to　succeed　accordingly；　for　upon　suspending　the　ball　on　the　tube　by　a　pack…thread　about　three　feet　long；　when　the　tube　had　been　excited　by　rubbing；　the　ivory　ball　attracted　and　repelled　the　leaf…brass　over　which　it　was　held　as　freely　as　it　had　done　when　it　was　suspended　on　sticks　or　wire；　as　did　also　a　ball　of　cork；　and　another　of　lead　that　weighed　one　pound　and　a　quarter。〃　Gray　next　attempted　to　determine　what　other　bodies　would　attract　the　bits　of　paper；　and　for　this　purpose　he　tried　coins；　pieces　of　metal；　and　even　a　tea…kettle；　〃both　empty　and　filled　with　hot　or　cold　water〃；　but　he　found　that　the　attractive　power　appeared　to　be　the　same　regardless　of　the　substance　used。　〃I　next　proceeded；〃　he　continues；　〃to　try　at　what　greater　distances　the　electric　virtues　might　be　carried；　and　having　by　me　a　hollow　walking…cane；　which　I　suppose　was　part　of　a　fishing…rod；　two　feet　seven　inches　long；　I　cut　the　great　end　of　it　to　fit　into　the　bore　of　the　tube；　into　which　it　went　about　five　inches；　then　when　the　cane　was　put　into　the　end　of　the　tube；　and　this　excited；　the　cane　drew　the　leaf…brass　to　the　height　of　more　than　two　inches；　as　did　also　the　ivory　ball；　when　by　a　cork　and　stick　it　had　been　fixed　to　the　end　of　the　cane。。。。　With　several　pieces　of　Spanish　cane　and　fir…sticks　I　afterwards　made　a　rod；　which；　together　with　the　tube；　was　somewhat　more　than　eighteen　feet　long；　which　was　the　greatest　length　I　could　conveniently　use　in　my　chamber；　and　found　the　attraction　very　nearly；　if　not　altogether；　as　strong　as　when　the　ball　was　placed　on　the　shorter　rods。〃　This　experiment　exhausted　the　capacity　of　his　small　room；　but　on　going　to　the　country　a　little　later　he　was　able　to　continue　his　experiments。　〃To　a　pole　of　eighteen　feet　there　was　tied　a　line　of　thirty…four　feet　in　length；　so　that　the　pole　and　line　together　were　fifty…two　feet。　With　the　pole　and　tube　I　stood　in　the　balcony；　the　assistant　below　in　the　court；　where　he　held　the　board　with　the　leaf…brass　on　it。　Then　the　tube　being　excited；　as　usual；　the　electric　virtue　passed　from　the　tube　up　the　pole　and　down　the　line　to　the　ivory　ball；　which　attracted　the　leaf…brass；　and　as　the　ball　passed　over　it　in　its　vibrations　the　leaf…brass　would　follow　it　till　it　was　carried　off　the　board。〃　Gray　next　attempted　to　send　the　electricity　over　a　line　suspended　horizontally。　To　do　this　he　suspended　the　pack…thread　by　pieces　of　string　looped　over　nails　driven　into　beams　for　that　purpose。　But　when　thus　suspended　he　found　that　the　ivory　ball　no　longer　excited　the　leaf…brass；　and　he　guessed　correctly　that　the　explanation　of　this　lay　in　the　fact　that　〃when　the　electric　virtue　came　to　the　loop　that　was　suspended　on　the　beam　it　went　up　the　same　to　the　beam；〃　none　of　it　reaching　the　ball。　As　we　shall　see　from　what　follows；　however；　Gray　had　not　as　yet　determined　that　certain　substances　will　conduct　electricity　while　others　will　not。　But　by　a　lucky　accident　he　made　the　discovery　that　silk；　for　example；　was　a　poor　conductor；　and　could　be　turned　to　account　in　insulating　the　conducting…cord。　A　certain　Mr。　Wheler　had　become　much　interested　in　the　old　pensioner　and　his　work；　and；　as　a　guest　at　the　Wheler　house；　Gray　had　been　repeating　some　of　his　former　experiments　with　the　fishing…rod；　line；　and　ivory　ball。　He　had　finally　exhausted　the　heights　from　which　these　experiments　could　be　made　by　climbing　to　the　clock…tower　and　exciting　bits　of　leaf…brass　on　the　ground　below。　〃As　we　had　no　greater　heights　here；〃　he　says；　〃Mr。　Wheler　was　desirous　to　try　whether　we　could　not　carry　the　electric　virtue　horizontally。　I　then　told　him　of　the　attempt　I　had　made　with　that　design；　but　without　success；　telling　him　the　method　and　materials　made　use　of；　as　mentioned　above。　He　then　proposed　a　silk　line　to　support　the　line　by　which　the　electric　virtue　was　to　pass。　I　told　him　it　might　do　better　upon　account　of　its　smallness；　so　that　there　would　be　less　virtue　carried　from　the　line　of　communication。　〃The　first　experiment　was　made　in　the　matted　gallery；　July　2；　1729；　about　ten　in　the　morning。　About　four　feet　from　the　end　of　the　gallery　there　was　a　cross　line　that　was　fixed　by　its　ends　to　each　side　of　the　gallery　by　two　nails；　the　middle　part　of　the　line　was　silk；　the　rest　at　each　end　pack…thread；　then　the　line　to　which　the　ivory　ball　was　hung　and　by　which　the　electric　virtue　was　to　be　conveyed　to　it　from　the　tube；　being　eighty　and　one…half　feet　in　length；　was　laid　on　the　cross　silk　line；　so　that　the　ball　hung　about　nine　feet　below　it。　Then　the　other　end　of　the　line　was　by　a　loop　suspended　on　the　glass　cane；　and　the　leaf…brass　held　under　the　ball　on　a　piece　of　white　paper；　when；　the　tube　being　rubbed；　the　ball　attracted　the　leaf…brass；　and　kept　it　suspended　on　it　for　some　time。〃　This　experiment　succeeded　so　well　that　the　string　was　lengthened　until　it　was　some　two　hundred　and　ninety…three　feet　long；　and　still　the　attractive　force　continued；　apparently　as　strong　as　ever。　On　lengthening　the　string　still　more；　however；　the　extra　weight　proved　too　much　for　the　strength　of　the　silk　suspending…thread。　〃Upon　this；〃　says　Gray；　〃having　brought　with　me　both　brass　and　iron　wire；　instead　of　the　silk　we　put　up　small　iron　wire；　but　this　was　too　weak　to　bear　the　weight　of　the　line。　We　then　took　brass　wire　of　a　somewhat　larger　size　than　that　of　iron。　This　supported　our　line　of　communication；　but　though　the　tube　was　well　rubbed；　yet　there　was　not　the　least　motion　or　attraction　given　by　the　ball；　neither　with　the　great　tube；　which　we　made　use　of　when　we　found　the　small　solid　cane　to　be　ineffectual；　by　which　we　were　now　convinced　that　the　success　we　had　before　depended　upon　the　lines　that　supported　the　line　of　communication　being　silk；　and　not　upon　their　being　small；　as　before　trial　I　had　imagined　it　might　be；　the　same　effect　happening　here　as　it　did　when　the　line　that　is　to　convey　the　electric　virtue　is　supported　by　pack…thread。〃　Soon　after　this　Gray　and　his　host　suspended　a　pack…thread　six　hundred　and　sixty…six　feet　long　on　poles　across　a　field；　these　poles　being　slightly　inclined　so　that　the　thread　could　be　suspended　from　the　top　by　small　silk　cords；　thus　securing　the　necessary　insulation。　This　pack…thread　line；　suspended　upon　poles　along　which　Gray　was　able　to　transmit　the　electricity；　is　very　suggestive　of　the　modern　telegraph；　but　the　idea　of　signalling　or　making　use　of　it　for　communicating　in　any　way　seems　not　to　have　occurred　to　any　one　at　that　time。　Even　the　successors　of　Gray　who　constructed　lines　some　thousands　of　feet　long　made　no　attempt　to　use　them　for　anything　but　experimental　purposessimply　to　test　the　distances　that　the　current　could　be　sent。　Nevertheless；　Gray　should　probably　be　credited　with　the　discovery　of　two　of　the　most　important　properties　of　electricitythat　it　can　be　conducted　and　insulated；　although；　as　we　have　seen；　Gilbert　and　Von　Guericke　had　an　inkling　of　both　these　properties。

EXPERIMENTS　OF　CISTERNAY　DUFAY　So　far　England　had　produced　the　two　foremost　workers　in　electricity。　It　was　now　France's　turn　to　take　a　hand；　and；　through　the　efforts　of　Charles　Francois　de　Cisternay　Dufay；　to　advance　the　science　of　electricity　very　materially。　Dufay　was　a　highly　educated　savant；　who　had　been　soldier　and　diplomat　betimes；　but　whose　versatility　and　ability　as　a　scientist　is　shown　by　the　fact　that　he　was　the　only　man　who　had　ever　contributed　to　the　annals　of　the　academy　investigations　in　every　one　of　the　six　subjects　admitted　by　that　institution　as　worthy　of　recognition。　Dufay　upheld　his　reputation　in　this　new　field　of　science；　making　many　discoveries　and　correcting　many　mistakes　of　former　observers。　In　this　work　also　he　proved　himself　a　great　diplomat　by　remaining　on　terms　of　intimate　friendship　with　Dr。　Graya　thing　that　few　people　were　able　to　do。　Almost　his　first　step　was　to　overthrow　the　belief　that　certain　bodies　are　〃electrics〃　and　others　〃non…electrics〃that　is；　that　some　substances　when　rubbed　show　certain　peculiarities　in　attracting　pieces　of　paper　and　foil　which　others　do　not。　Dufay　proved　that　all　bodies　possess　this　quality　in　a　certain　degree。　〃I　have　found　that　all　bodies　（metallic；　soft；　or　fluid　ones　excepted）；〃　he　says；　〃may　be　made　electric　by　first　heating　them　more　or　less　and　then　rubbing　them　on　any　sort　of　cloth。　So　that　all　kinds　of　stones；　as　well　precious　as　common；　all　kinds　of　wood；　and；　in　general；　everything　that　I　have　made　trial　of；　became　electric　by　beating　and　rubbing；　except　such　bodies　as　grow　soft　by　beat；　as　the　gums；　which　dissolve　in　water；　glue；　and　such　like　substances。　'Tis　also　to　be　remarked　that　the　hardest　stones　or　marbles　require　more　chafing　or　heating　than　others；　and　that　the　same　rule　obtains　with　regard　to　the　woods；　so　that　box；　lignum　vitae；　and　such　others　must